This invention relates to tracheal tube assemblies of the kind having an outer tube and an inner cannula that is insertable within and removable from the outer tube.
With such assemblies, the inner cannula is removed and replaced periodically when secretions have built up on the cannula to an extent that there is a risk of blockage. Tracheal tube assemblies are described, for example, in U.S. Pat. No. 3,948,274, GB 2056285B, GB 1099277, GB 125754, WO 90/04992, FR 2539998A, DE 72467, DE 1268313, EP 0107779A, U.S. Pat. Nos. 4,817,598, 3,659,612, 4,009,720, 3,088,466, 4,315,545, 2,765,792, 3,169,529, 3,263,684, 3,334,631, 3,587,589, 3,688,774, 3,731,692, 3,889,688, 3,948,273, 3,973,569, 3,987,798, 4,033,353, 4,045,058, 4,235,229, 4,471,776, 4,593,690.
In order to ensure as large as possible bore through the assembly, the wall of the inner cannula must be as thin as possible and the external diameter of the inner cannula must be as close as possible to the internal diameter of the outer tube. This, however, increases the risk of kinking of the cannula on insertion, especially where friction with the outer tube causes a greater axial force to be exerted on the cannula. A further problem arises because, if the cannula is made of a low-friction material, these tend not to enable respiratory secretions to cling to them readily, thereby increasing the risk that secretions will become dislodged and fall into the bronchi and lead to infection.
It is an object of the present invention to provide an improved inner cannula for a tracheal tube assembly.